Enhancement of complementary metal oxide semiconductor (CMOS) circuit requires improvement in the performance of both the p-type metal oxide semiconductor field effect transistors (PMOSFETs) and n-type metal oxide semiconductor field effect transistors (NMOSFETs). While the same material and processing steps were shared between PMOSFETs and NMOSFETs in the past, recent trends in high performance PMOSFETs and NMOSFETs show increased use of different materials and different processing steps among the two types of transistors.
An example in which differences in the manufacture of the transistors are preferred is in the selection of the gate conductor material. In the case of an NMOSFET, it is preferred that a work function of a gate electrode material is close to a conduction band edge. In contrast, in the case of a PMOSFET, it is preferred that a work function of a gate electrode material is close to a balance band edge. Since the conduction band edge is separated by the balance band edge by a band gap in a semiconductor material, the work function of the gate electrode material for the PMOSFET needs to be different from the work function of the gate electrode material for the NMOSFET. Hence, the need arises to utilize two different gate electrode materials for a high performance CMOS circuit, in which one material is utilized for the gates of PMOSFETs while another material is utilized for the gates of the NMOSFETs.
Various CMOS device structures with two different gate electrode materials and methods of manufacturing the same have been known in the art. For example, Rhee et al., in U.S. Patent Application Publication No. 2002/0113294 discloses CMOS devices with doped silicon germanium alloy gate electrodes with differing concentration gradients of germanium between PMOSFET electrodes and NMOSFET electrodes. Similarly, Takayanagi et al., in U.S. Pat. No. 6,746,943, disclose compensation for differences in activation of p-type dopants and n-type dopants with polysilicon-germanium alloy material having different germanium concentrations between PMOSFET electrodes and NMOSFET electrodes. Further, Polishchuk et al., in U.S. Pat. No. 6,794,234 discloses CMOS devices in which PMOSFET gate electrodes comprise a first metal, while NMOSFET gate electrodes comprise a second metal. Some of the prior art listed above also enables use of at least one high-k dielectric material within metal gate structures.
Using one metal for one type of gate electrode and polysilicon for another type of gate electrode is an alternative to the above listed prior art. An advantage of such an approach is that process integration is less complex compared to integration schemes that utilize two metal gate materials since processing of each metal gate material tends to introduce challenges. At the same time, utilization of a metal gate offers a control mechanism for gate work function that is effective enough to achieve substantial improvement in the performance of one type of transistors.
Since the use of a metal gate electrode with a high-K dielectric often introduces additional challenging, and oftentimes costly, processing steps, improvement of device performance through use of a metal gate electrode needs to be evaluated against the cost of the additional processes. For example, performance of NMOSFETs may improve significantly with the use of a metal gate electrode and a high-K dielectric to justify the associated additional cost while improvement of performance of PMOSFETs may not be sufficient to justify associated incremental cost.
Therefore, there exists a need for integration schemes that employ a metal gate material and a high-K dielectric on one type of MOSFET while utilizing a polysilicon gate on the other type of MOSFET.
Furthermore, there exists a need for integration schemes that manufacture a high-K dielectric metal gate MOSFET and a polysilicon gate MOSFET on the same semiconductor substrate with as little additional process complexity and processing cost as possible.